The present invention relates to a new and improved apparatus for preventing the blowing out of the water supply from a water shaft or chute in a constant pressure air storage installation of a gas turbine power plant.
In its more particular aspects, the apparatus of the present development is of the type including a subterranean cavern for storing compressed air, conduits connecting the cavern and the gas turbine or a compressor thereof, respectively, and a compensation basin communicating with the water in the cavern through the water shaft or chute.
Constant pressure air storage installations for gas turbine power plants, in relation to air storage installations working with variable air pressure and having the same output or efficiency, and which air pressure can fluctuate within certain limits during operation, require only about one-third of the volume of the last-mentioned type of air storage installations. Hence, the structural expenditure and the erection costs of a cavern for a constant pressure storage installation are appreciably less than in the case of caverns working with variable air pressure.
To maintain the air pressure of a constant pressure storage installation constant there is used a water supply having a water column. The water supply compensates or balances the volume of air which has been consumed in the cavern. The water column opens into a free basin which is usually located at ground level and whose static pressure height corresponds to charging of the cavern, which with present day installations is located at a depth in the order of about 600 to 800 meters, corresponding to a static pressure of the water column of 60 to 80 bar, the water is forced upwards into the compensation basin, and during discharge of the cavern the water runs out of the basin into the cavern in order to ensure for the same pressure conditions.
During operation of air storage gas turbine installations it has been found that during charging of the cavern the water column which rises in the water shaft or chute containing the water supply releases the air which is dissolved in the water column. Hence, air bubbles are formed which, in upward direction, rapidly increase in volume. These air bubbles cause a density reduction within the water column, and thus, oscillations of such water column. In the extreme case, the water column could be blown out by the compressed air cushion, and thus, the cavern could completely empty.
In contrast to the normal velocity of dissolution of air in static water, the complete saturation occurs more rapidly in the cavern owing to the pronounced turbulence of the water during the charging and discharging operations, since after a very short amount of time all of the water particles come into contact with the air at the water surface. The quantity by weight of air which is taken-up by the water is proportional to the pressure, which, as stated, is in the order of between 60 and 80 bar. As to the thus dissolved air quantity the following comparison is informative:
At 1 bar air pressure and 10.degree. C. temperature 1 m.sup.3 water (=1000 kg) contains 29.2 grams air.
At 60 bar pressure and 10.degree. C. temperature 1 m.sup.3 water contains 1.7 kg air, in other words approximately 58 times the amount by weight. At atmospheric pressure such 1.7 kg air corresponds to about 1.32 m.sup.3. A water/air mixture which has expanded from 60 bar pressure to atmospheric pressure therefore contains more air than water in volume.
If water which has been saturated with air in this manner ascends upwardly out of the cavern, then due to the decreasing hydrostatic pressure the air is released and forms increasingly larger size bubbles. The average density of the water column thus becomes increasingly smaller and oscillations of the water column occur. If there are not undertaken appropriate measures this pressure drop can lead to a blow-out of the compressed air cushion along with the water column.
A heretofore known measure for preventing this blow-out resided in extending the water shaft or chute containing the water column along a U-shaped arc below the base of the cavern. The lowest point of the water shaft therefore must be located at least 0.15 h below the momentary water level within the cavern, wherein h represents the effective pressure height, i.e. the difference between the geodetic height of the surface water level in the compensation or balancing basin and the cavern water level.
With h=600 meters this would mean that the already 600 meter long water shaft must be guided downwardly by still at least an additional 90 meters, and specifically twice resulting in an impermissibly great increase in the costs of the construction.
Other suggestions aim at totally or partially preventing contact between the air and water by means of a protective layer placed upon the water surface. The protective layer, for example, may be a foil, a layer of a very slowly evaporating liquid, or a layer of small buoyant bodies floating on the water and forming a coherent cover thereon. A further suggestion is intended to brake the rapid ascent of the water/air mixture in the water supply and to separate the air bubbles from the water by appropriate fixtures or facilities. The ideal method, namely completely preventing contact between the air and water at the water surface, is realized in practice only with very great difficulty. It still remains to be seen, whether the remaining methods can prevent, particularly after longer periods of shutdown, more or less spontaneous segregations of air in the water supply which generate oscillations in the water column. Such oscillations cause fluctuations in the pressure of the stored air which, primarily, can interfere with the compressor operation.
Recently a method and an apparatus operating in accordance therewith have been suggested for the purpose discussed herein (see European Patent Application No. 81200690.6). This method and apparatus are based upon the principle of continuously withdrawing by suction the water layer which is close to the surface of the water in the cavern and in which primarily the air is concentrated, through a tube system and by pumping the same to the water surface in the compensation basin where such water will release the dissolved air due to the lower pressure. The water which has been withdrawn is replaced from the water supply either continuously or intermittently by using de-aerated water from the compensation basin. Accordingly, only de-aerated water is present in the water shaft, since no air can diffuse from the cavern into the water shaft.
The apparatus for carrying out this method contains within the cavern a grate of tubes floating on the water surface, which grate is constituted by a number of perforated tubes, a header or collecting tube and a number of floats. The perforated tubes open into the header. The water is withdrawn from the layer close to the surface by means of a swinging or pendulum tube located in the cavern and is conveyed to the water layer close to the surface in the compensation basin through a drain or suction pipe fixed in the water shaft by means of a pump and through a further swinging or pendulum tube in the compensation basin where the withdrawn water containing air is de-aerated by a further grate of tubes.
It would have to be assumed that the suction device in such an installation is not required to be in continuous operation, rather it will suffice to activate the same during weekends and during inspection and servicing of the storage installation during which the store is not emptied. During normal working days, practically the entire water content of the cavern will be admitted to the compensation basin, and thus, will be de-aerated so that, in general, the suction device can be placed out of operation.
This concept is disadvantageous due to the presence of moving components or parts, such as the pump in which the occurrence of cavitation must be taken into account, the articulated connections or joints interconnecting the stationary and the swinging sections of the suction pipe or conduit which are subject to wear and, like the pump, require some maintenance. Additionally, the floating grate of tubes including the vertical guiding means thereof constitutes a structurally expensive and correspondingly costly system which, furthermore, is difficult to install.